44 DISPERSION OF SPORULES. 



true piiccinia is instantly detected, as well as 

 the dark-coloured spots under the cuticle, 

 which precede its rupture by the spores. 

 Moist seasons, damp situations, over-manured 

 lands, and lateness in the crops, are peculiarly 

 favourable to mildew, which almost always 

 appears in a chance plant of wheat that may 

 have vegetated on a manure-heap. Some say 

 this is invariably the case, but it is far too 

 loose an assertion. The rapidity with which 

 it sometimes spreads is astonishing ; only let 

 the circumstances be favourable, and millions 

 upon millions of sporules seem ready to enter 

 the stomata, and germinate beneath them. 

 The atmosphere is charged to an inconceivable 

 extejat with such invisible organs of reproduc- 

 tion. Fries declares the sporules to be so infi- 

 nite that they rise like thin smoke into the air 

 by evaporation, and are dispersed in innumer- 

 able ways ; as for instance, by. the attraction 

 of the sun, by insects, by wind, by elasticity, or 

 by adhesion. He asserts that in one individual 

 he calculated on good grounds, that there 

 were at least ten millions, if not more. Thus 

 a stoma can scarcely ever perform the function 

 of inhalation without taking in more or less of 

 these sporules ; and it is a happy circumstance 



